LONDON (Reuters) - Britain's Aberdeen Asset Management (>> Aberdeen Asset Management plc) has agreed to buy U.S. peer Flag Capital Management in a confidential deal that more than doubles the assets managed by its private equity unit and underscores its ambitions to expand in the United States.

Asset managers around the globe have been racing to bulk up their presence in alternative markets such as infrastructure, private debt and equity, hedge funds and real estate as investors hunt for yield in a world of low interest rates.

The deal to buy Flag, which manages around $6.3 billion (4 billion pounds) in assets, marks Aberdeen's first major takeover since it completed the purchase of Scottish Widows Investment Partnership from Lloyds Banking Group (>> Lloyds Banking Group PLC) in 2014.

The acquisition grows Aberdeen's private equity unit to roughly $15 billion of assets under management and the broader alternatives platform to $21.3 billion, the company said on Wednesday.

Flag manages a range of small-to-mid-cap private equity assets in the United States, as well as private equity in the Asia-Pacific region at end-December, it added.

Aberdeen had a strategy to boost its alternatives offering to institutional investors through dealmaking, said Aberdeen Chief Executive Martin Gilbert in the statement.

"FLAG meets this objective in two ways. Initially, it strengthens further our private market capability by bringing additional Asian expertise and new U.S. resource. This will also benefit our overall pan-alternatives platform. Secondly, FLAG deepens and expands our U.S. client base, which is a key growth market for Aberdeen."

Flag's funds were at low risk of client outflows, Aberdeen said, as once launched, each fund's revenue stream is defined, based on committed capital and a fixed fee schedule over the multiple-year life of the fund, which typically is set at 12 years.

Britain's second-biggest listed asset manager by market capitalisation said the transaction is expected to close in the third quarter of 2015, subject to regulatory approval.

RBC Capital Markets analyst Peter Lenardos said he believed the acquisition would be "incrementally positive" as Aberdeen expands both its pan-alternatives capability and presence in North America.

(Additional reporting by Carolyn Cohn, editing by Sinead Cruise)

By Simon Jessop